Method of producing light transducers



Patented June 1, 1954 METHOD OF PRODUCING LIGHT TRANSDUCERS Constantin S. Szegho,

Pakswer, Elmhurst, 111.,

land Corporation, a cor Chicago, and Serge assignors to The Rauporation of Illinois No Drawing. Application December 26, 1951, Serial No. 263,498

This invention relates to light transducers such as image converters and the like, and more particularly to a novel method for producing such transducers.

In the production of light transducers containing within a. common envelope a fluorescent screen and a photo-emissive cathode sensitized with an alkali-metal vapor such as caesium or rubidium, considerable difliculties have been en countered due to the deleterious action of the alkali-metal vapor on commonly used fluorescent screen materials such as zinc sulfide, zinc-cadmium sulfide, zinc silicate (willemite), and calcium tungstate. It has been found that such screen materials blacken in varying degrees and lose their fluorescent properties when exposed to the alkali-metal vapor. Consequently, special and costly precautions have been taken to prevent the caesium or rubidium vapor from reaching the screens; image tubes have been designed with elaborate shielding of the fluorescent screen or with parts, movable in vacuum, which carry the screen from a position protected from the vapor during the processing of the photo-cathode to a final position after sensitization.

It is an important object of the present invention to provide a novel method of producing light transducers of the type incorporating a fluorescent screen and a photo-emissive cathode within a common envelope in such a manner that the necessity of shielding of the fluorescent screen from the alkali-metal vapor during sensitization of the photo-emissive cathode is obviated.

It has been found that the deterioration of the commonly used fluorescent screen materials is caused-by a substitution of caesium or rubidium for the metal atoms of the fluorescent screen material with subsequent deposition of the metal on the screen. In accordance with the laws of physical chemistry, substitution will occur whenever the free energy of the resulting alkali-metal compound is greater than the free energy of the original fluorescent screen material. In accordance with the present invention, the fluorescent screen is composd of at least one fluorescent metal-oxide compound having a free energy of formation greater than that of any combination of the sensitizing alkali metal with any anion group contained in the screen material. In other words, the fluorescent screen is composed of either a single fluorescent metal-oxide, or a. mixture of such oxides, or a multiple oxide formed from two or more such single oxides, so long as the condition is satisfied that each component or constituent single oxide it may be shown that such 6 Claims. Cl. 3164) has a free energy of formation greater than that of any combination of the sensitizing alkali metal with any anion group contained in the screen material.

While the invention is applicable to the production of light transducers employing any type of photo-emissive cathode which is sensitized by exposure to an alkali-metal vapor, it is convenient for the purpose of explaining the invention to consider the production of a particular type of light transducer in which caesium vapor is employed for sensitization. The commonly employed antimony-caesium and silver-caesium oxide-caesium composite photosurfaces are in this category. In accordance with the present invention, the fluorescent screen of such a light transducer is composed essentially of at least one fluorescent metal-oxide compound having a free energy of formation greater than that of any combination of the caesium vapor with anyanion group contained in the screen material.

While complete data on the free energy of caesium oxide at 200 0., the temperature at which sensitization of the photo-emissive cathode is customarily achieved, is not readily available, it may be estimated with a satisfactory degree of approximation. The heat of formation of caesium oxide at room temperature is 164.4 kilo-v calories per gram molecule of oxygen. Because caesium is partially gaseous at 200 C., a correc tion of 5.2 kilo-calories per gram molecule of oxygen must be added. The entropy change of the reaction may be estimated at about 10 kilo calories per gram molecule of oxygen and must be subtracted from the corrected heat of formation. Thus, the resulting free energy of formation of caesium oxide from its components is approximately kilo-calories per gram molecule of oxygen.

Oxides having greater free energies than that of caesium oxide are not reduced by caesium vapor. Such oxides are, principal1y,-the lowest oxides of the elements berylium, magnesium, calcium, strontium, barium, aluminum, the rare earths (cerium, Samarium, europium, gadolinium, terbium, dysprosium, etc.) and boron, silicon, titanium, zirconium and thorium, oxides of chromium and manganese being borderline cases.

In general, the formation of double or multiple oxides increases the free energy over the sum of those of the constituent single oxides by about 8.5 kilo-calories per gram molecule of oxygen, although the formation of certain double or multiple oxides may be accompanied by a greater increase in free energy; for example, calcium silicats has a free energy of formation which exceeds the sum of the free energies of the constituent single oxides by about 21.25 kilo-calories per gram molecule of oxygen. Although complete data for the formation of caesium compounds is not available, alkali metals when forming double oxides such as silicates show a greater increase in free energy than other oxides. However, this additional increase does not exceed 20 to 30 kilo-calories per gram molecule of oxygen, so that multiple oxides having greater free energies than 180 to 190 kilo-calories per molecule of oxygen are not reduced by caesium vapor.

In accordance with the invention, the fluorescent screen of a light transducer embodying a caesium-sensitized photo-emi sive cathode is composed of at least one fluorescent metal-oxide compound having a free energy of formation greater than 160 kilo-calories per gram molecule of oxygen, or, in the event multiple oxides are employed, having a free energy of formation greater than 180 kilo-calories per gram molecule of oxygen. Of course, other mate'ials may be present in activator proportions. Farticularly good results have been achieved by forming the fluorescent screen of titanium-activated calcium silicate or calcium-magnesium silicate. when the fluorescent screen is formed of such materials, the photo-emissive cathode may be sensitized by exposure to caesium vapor without requimg the precaution of shielding the fluorescent sc een from the vapor and without substantially inipairing the sensitivity or the fluorescent screen.

Of course, photo-emissive cathodes which are sensitized by alkali-metal vapors other than caesium may be employed, in which case the fluorescent screen is composed or" at least one fluorescent metal-oxide compound having a free energy of formation greater than that of any combination of the sensitizing alkali metal with any anion group contained in the screen material. However, caesium is by far the most commonly employed sensitizing medium, and those screen materials which are suitable in light transducers employing caesium-sensitized photo-cathodes are generally suitable for use in tubes employing photo-cathodes which are sensitized by exposure to rubidium or other alkali-metal vapors.

Thus, the present invention provides a new and improved method of producing light transducers of the type incorporating a fluorescent screen and a photo-emissive cathode within a common envelope without shielding the fluorescent screen from the alkali-metal vapor of the photo-emissive cathode, thereby providing a simple solution to a problem of long standing in the art. No blackening or impairment of fluorescence is observed even after full exposure to and prolonged treatment in caesium vapor at elevated temperatures of the order of 200 C.

We claim:

1. The method of producing a light transducer which comprises: forming within an envelope a fluorescent screen consisting essentially of at least one of the fluorescent metal-oxide compounds having a free energy of formation greater than that of any combination of a predetermined alkali metal with any anion group contained in said compounds; forming within the same envelope a photo-emissive cathode; and exposing both said photo-emissive cathode and said. fluoduring sensitization 1 rescent screen to an atmosphere of said alkali metal in vapor phase to sensitize said photo-emissive cathode without substantially impairing the sensitivity or" said fluorescent screen.

2. The method of producing a light transducer which comprises: forming within an envelope a fluorescent screen consisting essentially of at least one of the fluorescent metal-oxide compounds having a, free energy of formation greater than that of any combination of caesium with any anion group contained in said compounds; forming within the same envelope a photo-emissive cathode; and exposing both said photoemissive cathode and said fluorescent screen to an atmosphere of caesium in vapor phase to sensitize said photo-emissive cathode without substantially impairing the sensitivity of said fluorescent screen.

3. I'he method of producing a light transducer which comprises: forming within an envelope at fluorescent screen consisting essentially of at least one of the fluorescent metal-oxide compounds having a free energy of formation greater than 160 kilo-calories per gram molecule of oxygen; forming within the same envelope a photoemissive cathode; and exposing both said photoemissive cathode and said fluorescent screen to an atmosphere of caesium in vapor phase to sonsitize said photo-emissive cathode without substantially impairing the sensitivity of said fluorescent screen.

4. The method of producing a light transducer which comprises; forming within an envelope at fluorescent screen consisting essentially of at least one of the fluorescent oxide compounds of the group of metals comprising beryllium, magnesium, calcium, barium, aluminum, boron, silicon, titanium, zirconium, thorium, and the rare earth metals; forming within the same envelope a photoemissive cathode; and exposing both said photo-emissive cathode and said fluorescent screen to an atmosphere of caesium vapor to sensitize said photo-emissive cathode without substantially impairing the sensitivity of said fluorescent screen.

5. The method of producing a light transducer which comprises: forming within an envelope a fluorescent screen consisting essentially of at least one of the fluorescent metal-oxide compounds having a free energy of formation greater than that of any combination of a predetermined alkali metal with any anion group contained in said compounds; forming within the same envelope a photo-emissive cathode; and exposing said photo-emissive cathode to an atmosphere of said alkali metal in vapor phase without shielding said fluorescent screen from said alkalimetal vapor to sensitize said, photo-emissive cathode without substantially impairing the sensitivity of said fluorescent screen.

6. The method of which comprises: forming within an envelope a fluorescent screen consisting essentially of titanium-activated calcium-magnesium silicate; forming within the same envelope at photo-emissive cathode; and exposing both said photo-emissive cathode and said fluorescent screen to an atmosphere of caesium vapor to sensitize said photoemissive cathode without substantially impairing the sensitivity of said fluorescent screen.

No references cited.

producing a light transducer 

1. THE METHOD PRODUCING A LIGHT TRANSDUCER WHICH COMPRISES: FORMING WITH AN ENVELOPE A FLUORESCENT SCREEN CONSISTING ESSENTIALLY OF AT LEAST ONE OF THE FLUORESCENT METAL-OXIDE COMPOUNDS HAVING A FREE ENERGY OF FORMATION GREATER THAN THAT OF ANY COMBINATION OF A PREDETERMINED ALKALI METAL WITH ANY ANION GROUP CONTAINED IN SAID COMPOUNDS; FORMING WITHIN THE SAME ENVELOPE A PHOTO-EMSSIVE CATHODE; AND EXPOSING BOTH SAID PHOTO-EMISSIVE CATHODE AND SAID FLUORESCENT SCREEN TO AN ATMOSPHERE OF SAID ALKALI METAL IN VAPOR PHASE TO SENSITIZED SAID PHOTO-EMIS SIVE CATHODE WITHOUT SUBSTANTIALLY IMPARING THE SENSITIVITY OF SAID FLUORESCENT SCREEN. 